This invention relates generally to lamp control and lamp switch circuits and more particularly, to electrical circuits for controlling and maintaining the appropriate balance of light output, end-to-end of a long multi-vapor arc lamp.
In the photoelectrophoretic imaging process, black and white or full color images are formed through the use of photoelectrophoresis. An extensive and detailed description of the photoelectrophoretic process is found in U.S. Pat. Nos. 3,384,488 and 3,384,565 to Tulagin and Carreira, 3,383,933 to Yeh and 3,384,566 to Clark, which disclose a system where photoelectrophoretic particles migrate in image configuration providing a visual image at one or both of two electrodes between which the particles suspended within an insulating carrier is placed. The particles are photosensitive and are believed to bear a net electrical charge while suspended which causes them to be attracted to one electrode and apparently undergo a net change in polarity upon exposure to activating electromagnetic radiation. The particles will migrate from one of the electrodes under the influence of an electric field through the liquid carrier to the other electrode.
The photoelectrophoretic imaging process is either monochromatic or polychromatic depending upon whether the photosensitive particles within the liquid carrier are responsive to the same or different portions of the light spectrum. A fullcolor polychromatic system is obtained, for example, by using cyan, magenta and yellow-colored particles which are responsive to red, green and blue light respectively.
It has been found that long multi-vapor lamps with their high efficiencies and controllable spectral outputs show great promise in producing optimum results in photographic system using artificial light. The most important factor in controlling the spectral energy distribution uniformity along the length of the lamp is the metal halide materials present in the vapor. For example, in the photoelectrophoretic imaging process using opaque or transparent originals, selecting appropriate metal halide materials and balancing their ratios to match photoelectrophoretic pigment sensitivities for the best process response is a major area of concern. Some long multi-vapor arc lamps in commercial use require about 2 mg. or mercury to facilitate starting. In connection with certain photoelectrophoretic imaging operations, the photoelectrophoretic process should not "see" the spectral emission lines of mercury vapor for optimum images. In other photoelectrophoretic imaging systems, mercury may be virtually absent from the lamps used, nevertheless, there are special characteristics and problems associated with them. For example, as soon as the lamp is ignited, the metal halide materials within the lamp tends to selectively migrate to one end of the lamp with a resultant end-to-end unbalance in light output. This migration of materials is called cataphoretic migration.
Other inventions have been discovered for controlling the operation of the lamp intensity of vapor lamps. One such process is disclosed in the patent to Dessoulavy et al, U.S. Pat. No. 3,514,667 issued May 26, 1970. This patent shows a circuit for controlling operation of a discharge tube which includes rectifier means for converting current from an alternating current source into direct current to be delivered to the tube. The rectifier includes controlled rectifier elements having control terminals or gates, and controllable supply of impulses operable to determine the portion of a cycle of alternating current supply during which direct current flows through the tube. However, this patent is not concerned with the drawbacks of cataphoretic migration in long multi-vapor arc lamps.
Accordingly, it is an object of this invention to provide an electrical control system for maintaining appropriate balance of light output along the length of long multi-vapor arc lamps.
Another object of this invention is to compensate for changes in spectral energy distribution of long multi-vapor arc lamps with operating time and variations from lamp to lamp.
A further object of this invention is to provide for effects of ambient temperature cooling of long multi-vapor arc lamps end-to-end.
It is another object of this invention to employ a lamp switching circuit to periodically reverse the direction of current flow through long multi-vapor arc lamps for use in photoelectrophoretic imaging systems.
Another object of this invention is to control the relative dwell times of the two polarities of alternating voltage supply source to long multi-vapor arc lamps in order to prevent the same migrations that can also be contributed to by differential end-to-end temperatures of the lamp and end-to-end imperfections in the lamp.
Another object of this invention is to provide control circuits for starting and operating long multi-vapor arc lamps.
Still a further object of this invention is to improve the application of long multi-vapor arc lamps to photoelectrophoretic imaging techniques.